Have you seen splinter cell 1-4 on GC? These games had to be extremely downgraded just to run on GC. Yes, GC had faster RAM, but still 50% less compared to xbox, and on top of that xbox had HDD, so developers could cache everything and build even bigger levels. Xbox also had more advanced GPU, (pixel shader, verte. shaders, shadow buffers), so developers were using shaders and buffer shadows pretty much everywhere.
It's not like you need pixel shaders to render amazing water reflections (or shadow buffers to render shadows), because GC could do it in software, but you need hardware to do it fast enough. Trying to recreate shaders on GC was expensive, these effects take away processing power from the CPU that could be used for other things, this is all because shaders are just instructions executed on the GPU rather than the CPU. On top of that effects like bump mapping requires a bump map texture for every real texture you want to apply it to, so you need more RAM for that. Because of that developers very rarely used bump mapping and other effects on GC. For example Luigis Mansion. The bump mapping is found only one place in the game, a brick texture.
The only game that use bump mapping on many textures was star wars rogue leader, but that was space shooter with 2D background scenery and small levels with simple ground texture, so they could spend more RAM resources on ships (bump mapping, self shadows).
Xbox also had 2x vertex shaders, and enabling vertex shaders has the effect of doubling triangle load
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Xbox was the most capable 6 gen console, not GC. All you need to do is look at some Xbox games like splinter cell 1-4, doom 3, Riddick looked like some early ps3/x360 games.